This invention relates in general to vehicle traction control systems and in particular to a traction control algorithm which compensates for operation upon a deformable road surface.
Many new vehicles include traction control systems for controlling the slippage of individual driven wheels upon low mu road surfaces during vehicle acceleration. Traction control systems are designed to maximize traction while providing optimized acceleration and stability. A number of conventional methods are available to implement a traction control system. Such methods include purely mechanical methods such as, for example, four wheel drive, limited slip differentials and locking differentials. Alternately, pure electronic methods also exist, such as control of an electronic engine throttle to reduce engine torque when slippage of the driven wheels is encountered. Also, there are know traction control systems that integrate electronics with mechanical operation For example, traction control is often integrated with an anti-lock brake system. The solenoid valves that control the amount of hydraulic pressure supplied to wheel brakes to reduce wheel slip during vehicle braking also can be used to brake a slipping driven wheel when excessive slippage occurs during vehicle acceleration. One such system reduces drive wheel slippage by retarding ignition timing, reducing fuel flow to the engine and cutting off selected engine cylinders in conjunction with application of individual wheel brakes.
Referring now to the drawings, FIG. 1 is a schematic diagram of a typical Traction Control (TC) system 10. The TC system 10 is illustrated for a rear wheel drive vehicle (not shown). The vehicle includes an engine 12 that provides power through a transmission 14 and differential 16 to a pair of driven wheels 18. As shown in FIG. 1, the vehicle also includes a pair of non-driven steerable front wheels 19. While a rear wheel drive vehicle is shown in FIG. 1, it will be appreciated that the following discussion also applies to front wheel drive vehicles. The vehicle further includes a powertrain control module (PCM) 20 that is electrically connected to the engine 12 and transmission 14 and an accelerator pedal 15. The PCM 20 is responsive to an electrical signal indicating a vehicle accelerator position to generate throttle position commands to control engine output torque and transmission shift commands. A typical PCM is described in U.S. Pat. No. 5,445,125.
The PCM 20 also is electrically connected to a TC controller 22. While the TC controller 22 is shown as a stand alone component in FIG. 1, it will be appreciated that the TC controller 22 can be integrated into an anti-lock brake system electronic control module or even the PCM 20. The TC controller 22 includes a TC microprocessor 24 that receives wheel speed signals from front wheel speed sensors 26 that monitor the speed of the front vehicle wheels 19 and from rear wheel speed sensors 28 that monitor the speed of the vehicle rear wheels 18.
The TC microprocessor 24 follows instructions contained in a TC algorithm that is stored in a read only memory (not shown). The microprocessor 24 is responsive to the TC algorithm instructions to sense the speed of the non-driven front wheel 19 calculate a vehicle speed from the sensed wheel speeds. The microprocessor 24 also senses the speed of each of the driven rear wheels 18. The microprocessor. 24 then determines the difference between the individual driven rear wheel speeds and the vehicle speed. The difference between the driven rear wheel speed and the vehicle speed is referred to as rear wheel slip.
A typical TC algorithm allows a limited amount of slip in the driven wheels before actuating traction control measures. Accordingly, the TC microprocessor 24 continuously monitors each of the rear wheels 18 for excessive slip, as defined by the rear wheel slip exceeding a predetermined slip threshold, Ts. The slip threshold Ts is selected in accordance with individual vehicle characteristics. A typical slip threshold Ts is two miles per hour. Upon detection that the slip of a driven wheel exceeds the slip threshold Ts, the microprocessor 24 actuates a traction control measure to reduce the excessive slippage of the driven wheel 18.
A typical TC algorithm is illustrated by the flow chart 30 shown in FIG. 2. The flow chart 30 is entered through block 32 periodically, as determined by the vehicle characteristics. For example, the TC algorithm may be entered every five milliseconds. The algorithm utilizes two flags, a Slip Control Flag, SCFLAG, and a Traction Control Flag, TCFLAG. The use of the two flags, SCFLAG and TCFLAG, will be described below; however, both flags are initially set False.
The algorithm proceeds to functional block 33, where a first wheel slip, S1, of one of the driven wheels 18 is determined. The algorithm then advances to decision block 34. In the decision block 34, the first wheel slip S1 is compared to the slip threshold Ts. If the first wheel slip S1 is greater than the slip threshold Ts, the algorithm branches to functional block 35 where the Slip Control Flag, SCFLAG, is set True. The algorithm then continues to decision block 36, which will be described below.
If, in the decision block 34, the first wheel slip S1 is less than, or equal to, the slip threshold Ts, the algorithm branches to functional block 37 where a second wheel slip, S2, of the other driven wheel is determined. The algorithm then advances to decision block 38. In the decision block 38, the second wheel slip S2 is compared to the slip threshold Ts. If the second wheel slip S2 is greater than the slip threshold Ts, the algorithm branches to functional block 35 and the Slip Control Flag, SCFLAG, is set True. The algorithm then continues to decision block 36.
In decision block 36, the TC microprocessor 24 determines if the current pass is the first pass through this portion of the algorithm. If the current pass is the first pass, the algorithm branches to functional block 39, where the Traction Control Flag, TCFLAG, is set True. The algorithm then proceeds to functional block 40. If the current pass is not the first pass in decision block 36, the algorithm proceeds directly to functional block 40.
In functional block 40, the TC microprocessor 24 activates a conventional traction control measure, such as the ones described above. For example the engine throttle can be partially closed and/or the brakes of the excessively slipping wheel applied. Additionally, the ignition timing of the engine may be retarded and/or some of the engine cylinders cut off. The algorithm then exits through block 42. Upon exiting the algorithm, the TC microprocessor 24 will wait for the predetermined time period and then reenter the algorithm through block 32.
If, in decision block 38, the second wheel slip S2 is less than, or equal to, the slip threshold Ts, the algorithm branches to functional block 44 where the Slip Control Flag, SCFLAG, is set False. This will occur when the traction control measures applied in functional block 40 cause the driven wheel slip to fall below the slip threshold, Ts, or when there is no excessive driven wheel slip. The algorithm then continues to decision block 45.
In decision block 45, the TC microprocessor 24 determines the status of the Traction Control Flag, TCFLAG. If the TCFLAG is true, the traction control has been previously activated. Although the driven wheels are no longer excessively slipping, it is desirable to allow the driven wheels to “spin-up” in speed to determine is the vehicle is still operating upon a low mu surface. Accordingly, the algorithm includes a traction control end delay timer, (not shown) which begins running when the driven wheel speed falls below the slip threshold, Ts. If the driven wheel slip does not again exceed the slip threshold, Ts, within the end delay, the traction control is deactivated and control of the vehicle speed is returned to the vehicle operator. A predetermined time period is used for the traction control end delay, which, in the preferred embodiment, is two seconds. Thus, if the TCFLAG is True in decision block 45, the algorithm transfers to decision block 46, where the TC end delay timer is checked. If the timer has not expired, the algorithm proceeds to functional block 47 where the torque being applied to the driven wheels is increased by reversing the measures applied in functional block 40. The increased torque causes the driven wheels to begin spinning-up. The algorithm then exits through block 42.
If, in decision block 46, the TC microprocessor 24 determines that the TC end delay timer has run, the algorithm branches to functional block 48 where the Traction Control Flag, TCFLAG, is set False. The algorithm then exits through block 42. Accordingly, during the next pass through the algorithm, if the driven wheel slip remains below the slip threshold, Ts, the algorithm will branch from decision block 45 to functional block 50 where full control of the vehicle speed is returned to the vehicle operator. The algorithm then exits through block 42.
A TC system that utilizes throttle control position is illustrated in FIGS. 3 through 5. In FIG. 3, the vehicle and drive wheel speeds are shown as a function of time. The status of the Slip Control and Traction Control Flags, SCFLAG and TCFLAG, are shown in FIGS. 4A and 4B, respectively, while the throttle opening position is shown in FIG. 5. At t1, the vehicle operator depresses the accelerator pedal 15 while the vehicle is on a slippery, or low mu, surface. For illustrative purposes, it is assumed that the initial accelerator pedal position calls for an 80 per cent throttle opening, as shown in FIG. 5. It will be appreciated that the following discussion also applies to other initial throttle openings.
Following the depression of the accelerator pedal 15, at least one of the driven wheels 18 begins to spin upon the low mu surface, as shown by the solid curve labeled 54 in FIG. 3, while the vehicle does not move. While the speed of only one driven wheel is illustrated in FIG. 3, it will be appreciated that the following discussion applies to both driven wheels. At t2, the driven wheel speed exceeds the slip threshold Ts, which is shown as a dashed line labeled 56 in FIG. 3. Accordingly, both Slip Control and Traction Control Flags are set True, as shown in FIGS. 4A and 4B. With both flags set True, the TC microprocessor 24 implements a TC measure, which, for illustrative purposes, consists of reducing the throttle opening to 25 per cent. It will be appreciated that other reduced throttle openings also can be utilized. Additionally, as described above, other conventional TC methods also can be implemented to reduce the slippage of the driven wheel. In response to the reduced throttle opening, the speed of the slipping driven wheel passes through a maximum and then begins to decrease.
At t3, the vehicle begins to move, as shown by the solid curve labeled 58 in FIG. 3. As the curve 58 representing the vehicle speed moves in an upward direction in FIG. 3, the TC threshold curve 56 also increases to maintain a difference between the threshold curve 56 and the vehicle speed curve 58 which is equal to the slip threshold Ts. The driven wheel speed 54 falls below the threshold curve 56 at t4. Accordingly, the Slip Control Flag is set False and torque supplied to the driven wheels is increased. For illustrative purposes, the increase of torque is shown by opening the throttle to 50 percent in FIG. 5. In the preferred embodiment, the amount of throttle opening is a function of the road surface mu, with a greater opening being used on higher mu surfaces. It will be appreciated that other throttle settings or other methods can be used to increase torque. The driven wheel speed then begins to increase and again crosses over the threshold curve 56 at t5. Accordingly, the Slip Control Flag is again set True and the Throttle reduced to a 25 per cent opening, as shown in FIGS. 4A and 5.
The TC continues to operate as described above with the driven wheel speed oscillating above and below the slip threshold curve 56 until the vehicle operator partially lifts his foot on the accelerator pedal 15 at t7. Due to operation of the TC system, the altered throttle position does not appear in FIG. 5 until t8. However, the vehicle speed reaches a constant value at t9 and the slipping driven wheel speed eventually becomes equal to the vehicle speed, as shown to the right in FIG. 3. Although the Slip Control Flag becomes false at t11, the Traction Control Flag remains true until t12, when the flag is set False. This represents the end delay period for the traction control.